The invention is in the field of metal-oxide-semiconductor (MOS) field-effect devices, and relates more specifically to lateral trench-gate bipolar transistor (LTGBT) devices suitable for power integrated circuit applications.
In the field of power integrated circuits, device designers seek to achieve devices with low on-resistance, fast switching times and high breakdown voltage. One category of devices which has exhibited considerable promise in these areas is the insulated-gate bipolar transistor (IGBT), a class of devices which incorporates the insulated-gate structure of a conventional majority-carrier MOS device, and additionally uses minority carrier conduction for conductivity modulation. Representative prior-art IGBT devices are shown in EP 0 111 803, EP 0 372 391 and DE 3820677. Examples of other types of structures, using various trench configurations, are shown in U.S. Pat. No. 4,546,367 and EP 0 047 392.
Although minority-carrier IGBT devices have received considerable attention because of their favorable performance characteristics when used in high-power applications, these devices have heretofore suffered from an important drawback. In particular, conductivity-modulated power devices tend to "latch up" at high current densities, a problem which is exacerbated when high voltages are also present. Since high-current and/or high voltage conditions are always encountered in power devices, a way is needed to provide conductivity-modulated power devices with high resistance to latch up. Absent such an improvement, these devices will tend to "latch up" in a manner similar to that exhibited by thyristors, such that they will be unable to be turned "off". This may result in temporary, or even permanent, damage to the devices.
A prior-art solution to this problem has been to increase the doping of the channel region in IGBT devices. This lowers the channel resistance, and therefore the voltage drop across the channel during conduction, resulting in a more latch-up resistant device. However, a major drawback of this technique is that the threshold voltage of the device is thereby increased, typically to a level where it is difficult to provide a sufficient gate turn-on voltage. Furthermore, this expedient only reduces, but does not eliminate, the problem.
Accordingly, a substantial need exists for a device which will incorporate the inherent advantages of conventional IGBT devices, while at the same time affording substantially improved resistance to latch up.